PhD candidate Melanie Rieback has been touring and explaining the RFIDGuardian. The device selectively blocks RFID signals and fits in your pocket but the goal is something small enough to embed in cellphones or PDAs. You can read all about it in Hongliang Wang's thesis and sign up to purchase their first models at the site.

This has previously been disscussed here and here but the thesis and hardware Beta test are new. Go get them!